As of Impulse v2.0, we use Shopify's product recommendation engine (read their docs here).
Before v2.0, related products only show up when you're on a collection-relative URL, such as /collections/your-collection/products/your-product. You can only get to this URL if you navigate to a product through a collection (or featured collection homepage section).
In all versions you can overwrite the standard functionality and specifically select related products on your individual product pages with the help of utility tags.
How does it work?
Impulse's utility tags allow you to show a related collection of your choosing, even when navigating directly to the product URL (/products/your-product).
In v2.0, make sure to enable "Product recommendations" in your theme settings. Before v2.0, make sure "Show related products" is enabled in your Product pages settings. Both are enabled by default.
1. Get your collection handle
First, you'll need the collection handle that you want to feature on your product. From your /admin/collections page, click "Edit website SEO" near the bottom of the page.
Select the text in the last field after /collections/.
2. Add the utility tag to your product
Navigate to the product you want to add the above collection to as related products. Add your utility tag in the format _related_collection-handle.
In this example it would be _related_menswear.